Around the chin you need to take matters in your hand, literally. Use a light marker or clear shaving gel to mark the shape of your beard. Make sure that both sides are even by using a ruler for the markings. The third step is thus styling out with the help of scissors and trimmer. A razor may not be able to establish the right style and thus a trimmer is a better option.
The fourth step is to then grow out the beard beyond the jawline. While you do this make sure you do not narrow in the bottom of beard like you usually do for all other beard styles. The bottom is not tapered and rather flat. To achieve this look you will have to allow your beard to grow out even from the upper neck region. this will provide the support and aggression for the flat bottom style.